This invention relates to a single mode optical fiber designed for connection to a laser. The fiber is transmissive at the laser""s pump wavelength while strongly attenuating longer wavelengths. More particularly, the optical fiber has a depressed inner clad index profile and may be combined with a photosensitive core that permits writing of reflective gratings for laser stabilization. The fiber core is or can be substantially mode field matched to standard fibers used in optical communications devices to minimize splice losses.
The rapid growth of fiber optic telecommnunications in recent years has led to an increasing need for technologies that support increased data transmission rates. Many such systems employ single mode optical fibers, erbium-doped fiber laser light sources, and erbium doped fiber amplifiers (EDFA) operating in a dense wavelength division multiplexing (DWDM) mode. Operating at closely spaced wavelengths primarily in the central erbium gain band (C band) between 1530 nm-1560 nm in the near infrared, these systems can communicate as many as 100 optical signal channels. (Recent work has attempted to utilize the adjacent shorter wavelength xe2x80x9cS bandxe2x80x9d and longer wavelength xe2x80x9cL bandxe2x80x9d regions of the erbium gain spectrum.)
In order for the erbium atoms in an erbium-doped fiber laser or erbium-doped fiber amplifier to emit photons at 1530 nm-1560 nm, they must first absorb photons with a wavelength shorter than (or equal to) the emission wavelength. These shorter wavelength photons typically come from a xe2x80x9cpump laser,xe2x80x9d such as a semiconductor laser. In most systems, a pump laser which emits light of wavelengths around 980 nm or 1480 nm is used.
Lasers and optical amplifiers are sensitive to light that enters the laser cavity from reflections or other sources. This effect can be used to stabilize the operating wavelength of a laser, but generally the intrusion of light will destabilize the operating wavelength. In order to stabilize the operating wavelength, the reflected light must be within a specific narrow wavelength range. A weak external Bragg grating is sometimes provided to reflect some light back into the cavity thereby stabilizing the laser. Destabilization of the laser diode can occur if uncontrolled light (e.g., from reflections or from other light sources) is allowed to enter the laser cavity. Often isolators are employed to prevent such destabilizing light from reaching the laser diode.
A Bragg grating is written in a section of optical fiber by creating a periodic modulation in the refractive index of the fiber core. They are generally produced by exposing a photosensitive fiber to a periodic pattern of ultraviolet (UV) light. Their fundamental property is to reflect light over a narrow spectral range centered at a resonant wavelength. Bragg gratings can be used to reflect, filter or disperse light within an optical fiber.
Long-period gratings can also be used to provide wavelength dependent loss. A long-period grating couples optical power between two copropagating modes with very low back reflections. A long-period grating typically comprises a length of optical fiber wherein a plurality of refractive index perturbations are spaced along the fiber by a periodic distance which is large compared to the wavelength of the transmitted light. Long-period fiber grating devices selectively remove light at specific wavelengths by mode conversion. In contrast with conventional Bragg gratings which reflect light, long-period gratings remove light without reflection by converting it from a guided mode to a non-guided mode.
Gratings are commonly written in optical fibers having a fiber core that is photosensitive to ultraviolet light. Photosensitivity is generally created by doping the fiber preform core with germanium during the preform fabrication process. Germanium is typically used to increase the refractive index of the fiber core. The core doped with GeO2 is often subjected to hydrogen or deuterium loading using any of several loading procedures. The hydrogen loaded fiber is then exposed to an optical interference pattern from a UV laser, which induces solid state photochemical reactions at the high light intensity xe2x80x9cbrightxe2x80x9d fringes of the interference pattern. These photochemical reactions produce xe2x80x9ccolor centersxe2x80x9d such as electrons trapped at defect sites in the glassy material; this causes a change in the refractive index at the locations exposed to the UV light. The hydrogen, in a sense, provides an additional source of electrons for this process, and typically increases the speed and magnitude of the refractive index change. However, the excess hydrogen slowly diffuses out of the optical fiber after the grating pattern is written, resulting in a slow change in the refractive indexes in the fiber causing a change in the reflectivity and the Bragg wavelength of the Bragg grating. Gratings are often heated for a period of time after they are written, in order to xe2x80x9cannealxe2x80x9d the grating and stabilize its reflectivity and Bragg wavelength at some value which will remain constant over time. Although a short term (10 minute) elevated temperature (300xc2x0 C.) annealing is common to most grating fabrication to pre-age the grating, additional long term annealing of several days at a low temperature (70xc2x0 C.) is commonly required in hydrogen loaded fiber gratings that have been recoated. (The protective polymer coatings on optical fibers are generally removed before writing a grating in the fiber.)
Aside from the light reflected to stabilize the operating wavelength, back reflections into lasers and amplifiers are unwanted. To prevent the unwanted back reflections, it is often necessary to install bulky and expensive optical isolators near lasers or optical amplifiers.
Single mode optical fibers having low loss transmission characteristics have existed for at least two decades. The low attenuation characteristics of the fibers are limited by scattering, absorption and bending losses in the fibers.
Optical fiber cores having a reduced refractive index and therefore requiring less core dopants were made feasible with the introduction of fiber with a depressed index cladding (DIC). These fibers are inherently lower in attenuation due to both the lower absorption losses in the less doped core and less scattering loss due to the tighter optical mode confinement. However, depressed index clad fibers suffer from a fundamental mode loss phenomena at longer wavelengths due to xe2x80x9ctunnelingxe2x80x9d through the depressed well. Very thick depressed cladding has been used to minimize the tunneling effect, but this cladding design makes production of these fibers less economical.
The apparent fundamental mode cutoff wavelength xcexfc is the smallest operating wavelength that a single mode fiber will propagate the fundamental mode where the loss is about 1 dB/m in a straight section of fiber. Beyond this wavelength, the 1st-order mode becomes lossy and radiates out of the fiber core.
The operating wavelength xcexop of the optical fiber should be transmitted with very little loss. As the wavelength of light propagating in the fiber becomes longer than the apparent fundamental mode cutoff wavelength xcexfc, the fundamental mode becomes increasingly lossy.
As light of test wavelengths successively longer than the apparent fundamental cutoff wavelength xcexfc are launched into the optical fiber, more and more power leaks through the fiber cladding. As the fundamental mode extends into the cladding material, it also becomes increasingly sensitive to bending loss even though near the xcexfc the bend sensitivity is actually less than in a standard step index fiber.
A combination of preferred index levels and core/DIC diameters were discovered that assured low losses in both the 1300 nm and 1550 nm operating window of silica-based optical fibers. Typically, these depressed index clad fibers resulted in less than 0.1 dB/km of additional loss at 1550 nm compared to the 1300 nm loss level. These designs were optimized for minimizing fundamental mode loss at the longer 1550 nm operating wavelengths thereby permitting the future upgrading of these links to the longer wavelength.
The lossy characteristic for depressed index clad fiber was considered an undesirable problem and the bulk of the research has generally been directed at fiber designs that minimize its effects.
One of the few fiber applications to take advantage of the lossy fundamental mode characteristic of these fibers is the polarizing fiber design of J. R. Simpson, et al. Journal of Lightwave Technology, 1983. In this design, a special birefringence-generating elliptical cladding structure having a depressed index is used to selectively attenuate one of the two orthogonal fundamental mode polarizations, resulting in the propagation of a single polarization mode. The xe2x80x9cfastxe2x80x9d axis polarization tunneled through the depressed cladding and radiated out to the lossy mode stripping coating, resulting in the propagation of a single polarization. The xe2x80x9cslowxe2x80x9d propagating mode also suffered from the same tunneling effect but at a slightly longer wavelength, thereby providing the polarizing wavelength window. Unfortunately, only several tens of nanometers of useable bandwidth has generally been possible with these fibers, which has limited their use to narrow line width laser sources. Also, this fiber design relied heavily on controlled tight bending of the fiber in order to tune the wavelength of the useful operating window.
Another approach to achieving a lossy fiber design has been through the incorporation of certain transition metal dopants in the core. Examples of these types of fibers are described in U.S. Pat. Nos. 4,881,793, 5,572,618 and 5,633,974. Because these dopants are not very wavelength specific in their attenuating characteristics and therefore provide relatively high attenuation at the desired low loss operating wavelengths, they are generally unsuitable for pigtailing applications.
Telecommunication systems can also suffer from a loss of signal when optical fibers are joined. To this end, pieces of optical fiber are usually joined together by fusion splicing with the fiber cores aligned. However, the losses are best minimized by reducing the transition of the electromagnetic field as light moves from one fiber to another. This is called mode matching, and is conceptually similar to impedance matching in electrical circuit design. To accomplish mode matching, the mode field diameters of the joined fibers are made equal.
In discussing the step index of optical fibers, it should be noted that because of the manufacturing processes employed, real optical fibers rarely have perfectly square refractive index profiles. There may be diffusion of dopants at the boundaries of the regions, and the core may exhibit a xe2x80x9cgermanium burnoff dipxe2x80x9d at its center. To account for this non-square index profile, there are standard algorithms for calculating an equivalent step index.
In accordance with the present invention, a single mode optical fiber is provided that operates with very low attenuation at the pump wavelength while providing very high attenuation at certain longer wavelengths. Typically, the attenuation is less than about 0.050 dB/meter at the pump wavelength and greater than about 1 dB/meter at the absorption wavelength.
The optical fiber includes a core region that is photosensitive, thereby permitting the writing of reflective gratings in the fiber for pump stabilization. The photosensitivity can be made sufficient to eliminate the need for H2 or D2 loading or other photosensitizing processes prior to the grating writing process.
The fiber can be easily spliced or connected to common wavelength selective couplers and other pigtail fibers with minimal losses.
These attributes are obtained in a DIC single mode fiber design having a core doped with photosensitizing compositions and having a mode field diameter that closely matches or can be made to closely match adjoining fibers. More specifically, the fiber design has a raised index between about 0.000 and 0.010, a depressed index between about 0.002 and 0.015 and a ratio of DIC diameter to core diameter within the range of about 2 to about 10, preferably within the range of about 2 to about 7, more preferably within the range of about 2 to about 5.5, and most preferably within the range of about 2.6 to about 5.1. Photosensitizing core compositions may include GeO2/SiO2 and GeO2 co-doped with B2O3/SiO2. The DIC composition may be a fluorine doped silica composition or a boron doped silica composition.